Generator electrode, coulometry

Controlled-current coulometry is also called coulometric titration. An apparatus for controlled-current coulometry is shown in Fig. 5.35 for the case of determination of an acid. It consists of a constant current source, a timer, an end-point detector (pH meter), and a titration cell, which contains a generating electrode, a counter electrode in a diaphragm, and two electrodes for pH detection. The timer... 

Fig. 5.35 Apparatus for controlled-current coulometry. The case for neutralization titration of an acid by internal cathodic generation of OhT. GE generating electrode CE counter electrode.

The applied current must be 1000 times the residual current to achieve a current efficiency of 99.9%. In many cases, a ratio of applied current to a residual current of 103 is reasonable for applied currents down to about 10 pA using generator electrode areas of about 0.1 cm2. Currents in excess of a few hundred milliamperes are seldom used in constant-current coulometry because the solubility limit of the precursor is reached and/or the experiment may be over too quickly to permit accurate measurement of the time. Heating effects (i2R) are also a problem when high currents are used. 

Maintenance of the systems is crucial to proper operation electrodes and reaction vials or chambers must be kept scrupulously clean and the proper shape and size of the Ag" -generating electrodes must be maintained. Only 12 of 5400 laboratories reported in the 2003 CAP survey reported CP results using coulometry However, some laboratories still maintain these instruments as backups and for sweat analysis,... 

Some typical important industrial applications of coulometry include the continuous monitoring of mercaptan concentration in the materials used in rubber manufacture. The sample continuously reacts with bromine, which is reduced to bromide. A third electrode measures the potential of B12 vs. Br and, based on the measurement, automatically regulates the coulometric generation of the bromine. Coulometry is used in commercial instruments for the continuous analysis and process control of the production of chlorinated hydrocarbons. The chlorinated hydrocarbons are passed through a hot furnace, which converts the organic chloride to HCl. The latter is dissolved in water and the Cl titrated with Ag" ". The Ag" " is generated by coulometry from a sUver electrode, Ag°. It is necessary for the sample flow rate to be constant at all times. Integration of the coulometric current needed to oxidize the silver to silver ion results in a measurement of the Cl concentration. 

Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is quantitatively oxidized or reduced at the working electrode or reacts quantitatively with a reagent generated at the working electrode. There are two forms of coulometry controlled-potential coulometry, in which a constant potential is applied to the electrochemical cell, and controlled-current coulometry, in which a constant current is passed through the electrochemical cell. 

Coulometry measures the amount of cunent flowing dirough a solution in an electrochemical oxidation or reduction reaction and is capable of measuring at ppm or even ppb levels of reactive gases. Thus a sample of ambient air is drawn through an electrolyte in a cell and the required amount of reactant is generated at the electrode. This technique tends to be non-specific, but selectivity can be enhanced by adjustment of pH and electrolyte composition, and by incorporation of filters to remove interfering species. 

Electrical methods of analysis (apart from electrogravimetry referred to above) involve the measurement of current, voltage or resistance in relation to the concentration of a certain species in solution. Techniques which can be included under this general heading are (i) voltammetry (measurement of current at a micro-electrode at a specified voltage) (ii) coulometry (measurement of current and time needed to complete an electrochemical reaction or to generate sufficient material to react completely with a specified reagent) (iii) potentiometry (measurement of the potential of an electrode in equilibrium with an ion to be determined) (iv) conductimetry (measurement of the electrical conductivity of a solution). 

In electroanalysis, coulometry is an important method in which the analyte is specifically and completely converted via a direct or indirect electrolysis, and the amount of electricity (in coulombs) consumed thereby is measured. According to this definition there are two alternatives (1) the analyte participates in the electrode reaction (primary or direct electrolysis), or (2) the analyte reacts with the reagent, generated (secondary or indirect electrolysis) either internally or externally. 

Among electrochemical techniques,cyclic voltammetry (CV) utilizes a small stationary electrode, typically platinum, in an unstirred solution. The oxidation products are formed near the anode the bulk of the electrolyte solution remains unchanged. The cyclic voltammogram, showing current as a function of applied potential, differentiates between one- and two-electron redox reactions. For reversible redox reactions, the peak potential reveals the half-wave potential peak potentials of nonreversible redox reactions provide qualitative comparisons. Controlled-potential electrolysis or coulometry can generate radical ions for smdy by optical or ESR spectroscopy. 

In the coulometric version of the instrument, the iodine necessary for reaction is generated electrochemically by applying electrical pulses to the electrode. In this case, a modified Karl Fischer reagent is used which contains iodide instead of iodine. An iodide solution is in contact with the anode in one compartment of the electrolytic cell, which has a diaphragm between the anodic and cathodic regions (Fig. 19.11). Thus, the volume of reagent, which is the indicator in the classical titration method, is replaced by a more precise quantity of current, obtained coulometrieally. 

The Karl Fischer titration of water uses a buret to deliver reagent or coulometry to generate reagent. In bipotentiometric endpoint detection, the voltage needed to maintain a constant current between two Pt electrodes is measured. The voltage changes abruptly at the equivalence point, when one member of a redox couple is either created or destroyed. 

As an alternative for the external application of a stepwise change in pH, as described above, titrant can be added by coulometry. Coulometric generation of protons or hydroxyl ions is possible by sending a current between a noble metal actuator electrode, situated closely around the ISFET gate, and a distant counter electrode 115]. 

Constant-potential coulometry [37,49] is presently employed to some extent for determining serum iron. Current generated at two electrodes with potentials of 460 and 325 mV is measured. Copper is reduced at both of them, while iron reduction occurs at one electrode (325 mV), so that copper can be blanked out. Iron-dextran (Imferon), sulfasalazine, and pyrazinamide interfere in this assay [37,49]. Iron-dextran is not detected in the dry-film Kodak assay for iron [49,55]. 

It is possible to deposit metals onto an electrode quantitatively, using the technique of electrogravimetiy, although this is not often used. Electrolysis may also be used to generate reagents to react with analytes, and this is referred to as coulometry. 

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